D-C Power supply circuit with high power factor

ABSTRACT

A d-c power supply is described which is energized from a relatively low frequency power source and produces a d-c output voltage with low ripple and high power factor. An inductance-capacitance circuit is connected in series with the a-c source and the a-c side of the rectifying element. A filter capacitor is connected to the d-c side of the rectifying element. The inductance-capacitance circuit resonates at a frequency of about three to six times the frequency of the input power supply. A plurality of circuits for carrying out the invention are disclosed.

BACKGROUND OF THE INVENTION

This invention relates to a d-c power supply, and more specificallyrelates to a novel circuit arrangement for a d-c power supply whichproduces low ripple d-c from a relatively low frequency a-c source,wherein the converter has an exceptionally high power factor with theuse of relatively few and inexpensive components.

Numerous power supplies are known for supplying a relatively high leveld-c voltage such as 300 V from a relatively low frequency a-c power linewhich might have a frequency of 50 Hz or 60 Hz. Such devices have almostuniversal application. One particular application of such a device isshown in co-pending application Ser. No. 966,604 filed Dec. 5, 1978, inwhich a high frequency converter is driven from the d-c output of arectifier network which is, in turn, powered from the relatively lowfrequency of the a-c mains in a building or the like. Conversion of a-cpower to low ripple d-c power for input to a high frequency convertercan be obtained with many known circuits. However, a relativelyinexpensive rectifier network is desired which can convert power to d-cat reasonable cost and in such a way that does not unduly load buildingwire or the power generating station that supplies the system.

Thus, the device preferably should be inexpensive in construction, easyto manufacture and reliable and should have a relatively high powerfactor.

The power factor of a rectifying network will be maximized when thephase difference between the line voltage and line current is minimaland further when the duty cycle of the line current is maximized. Theduty cycle of the line current is the per unit time that current flowsfrom the line during each half cycle of line voltage.

To understandd why a maximum duty cycle will maximize power factor,consider that the average power delivered from the line is: ##EQU1##where v is instantaneous voltage, i(t) is instantaneous current, t istime, V is rms voltage, and T is the period.

The effect of i(t) is, therefore, averaged over the cycle. However, thepower factor is the ratio of the average power P to the volt ampereproduct V×I, where I is the rms current defined as: ##EQU2## The rmscurrent I increases for a lower duty cycle because the waveform of i(t)is more peaked or has a shorter conduction period.

Many prior art rectifier circuits will be seen to have a poor powerfactor because their conduction time is short.

BRIEF DESCRIPTION OF THE PRESENT INVENTION

In accordance with the present invention, a novel rectifier circuit isprovided which is arranged to force the input line current to flow formost of the time during each half cycle so that the duty cycle of thecurrent is maximized. This is done through the use of a resonant circuitconnected on the a-c side of the rectifier element which is connected tothe d-c load circuit. This resonant circuit resonates at a frequency ofabout three to six times the frequency of the input source. The use ofthis resonant circuit maximizes the duty cycle of the line current andthus the power factor.

The preferred resonant circuit is an inductance-capacitance circuit withthe inductance in series with the a-c line and capacitance connectedacross the a-c lines and between the inductor and the rectifier bridge.The circuit operates to maintain the rectifier bridge in conduction formost of each half cycle, while also shaping the line current to a nearsinusoid. The result is a maximum duty cycle and power factor. Thevalues of the capacitor and inductor are chosen to best satisfy thisoperation. If improper values are chosen, or if the capacitance istotally eliminated, the inductor will not be able to maintain therectifier bridge in conduction over most of the half cycle; also, thecurrent would be out of phase with the voltage. Thus the power factorwould be much lower.

The novel circuit of the invention achieves the desired result ofmaximizing the duty cycle without greatly attenuating the d-c voltageand also produces a very low ripple voltage. Moreover, as will be shown,there is little line voltage distortion. The invention also permits theuse of a non-critical electrolytic filter capacitor in the rectifier d-ccircuit.

The novel arrangement of the invention also inherently limits in-rushcurrent to the circuit and prevents false tripping of the circuitbreaker when the system is turned on. The circuit also provides goodpower factor for wide variations of load current and thus is very usefulin applications in which the load current is to be widely varied as in alamp dimming application. The circuit also provides protection of thesemiconductor rectifier diodes from transient overvoltages in the supplyline. It will also be seen that the circuit uses all passive componentsand does not require complicated switching or control schemes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 3 show circuits representing prior art types of d-c powersupply arrangements.

FIG. 4 is a circuit diagram of a relatively high power factor circuit.

FIG. 5 is a circuit diagram of a power supply arrangement of the presentinvention.

FIG. 6a illustrates the voltage and current characteristics of thecircuit of FIG. 1.

FIG. 6b shows the voltage and current characteristics of the circuit ofFIG. 3.

FIG. 6c shows the voltage and current characteristics of the circuit ofFIG. 5 using the input tuned circuit of the invention and having arelatively high power factor and shows the voltage across the inputcapacitor.

FIGS. 7, 8 and 9 specifically illustrate current wave shape fordifferent resonant frequency ranges.

FIGS. 10 and 11 are circuit diagrams which assist the description of thetheory of operation of the circuit of the present invention.

FIG. 12 shows a second embodiment of the present invention wherein theinput voltage is rectified before conditioning by the tuned circuit ofthe invention and wherein the tuned circuit has a rectifier element inits output circuit.

FIGS. 13a to 13f show voltage and current wave shapes present in thecircuit of FIG. 12 plotted on a common time base.

FIGS. 14 and 15 show additional embodiments of the invention whichemploy voltage controlled inductors to further improve power factor.

FIGS. 16 to 19 show further embodiments of the invention wherein a highpower factor is maintained under extensive load changes.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring first to FIGS. 1 to 3, several typical, well-known powersupply circuits are disclosed. The simple arrangement of FIG. 1 is asingle phase full-wave bridge-connected rectifier 30 which is connectedto terminals 31 and 32 of a 60 Hz source. A large electrolytic capacitor33 is connected across the d-c output of the bridge 30 and the output isthen connected to a suitable d-c load.

It is well known that the circuit of FIG. 1 has a low duty cycle andvery low power factor and, therefore, unduly loads the a-c line. Thecircuit also has a disadvantage that large peak currents are drawn whichgreatly disrupt the a-c line voltage. Moreover, there will be a largein-rush current to charge the capacitor when the circuit is first turnedon which might falsely trip branch protection devices.

The circuit of FIG. 1 is frequently modified as shown in FIGS. 2 and 3by the addition of a large inductor 34 either in the d-c side of therectifier 30 as in FIG. 2 or in the a-c side as in FIG. 3. As will belater seen, this inductor is much larger than the one which is used inconnection with the present invention. When the inductor is introducedon the d-c side as shown in FIG. 2, the line current approaches a squarewave, whose power factor must be less than 0.9. When the inductor isintroduced on the a-c side, the power factor becomes progressively loweras the inductance is increased.

Another disadvantage of the circuit of FIG. 3 is that the d-c voltagewill be attenuated so that, for a given power output, the output currentmust be relatively high. In any converter, losses are mostly due tocurrent and, therefore, the efficiency of the circuit is reduced.

A circuit which improves power factor is that shown in FIG. 4. Thecircuit of FIG. 4 has two bridge rectifiers 40 and 41. The bridgerectifier 40 has an inductor 42 connected in series with one of the a-cterminals and a capacitor 43 is connected across the a-c terminals ofthe bridge.

An inductor 44 is connected across the a-c terminals of bridge 41 andcapacitor 45 is connected in series with the a-c bridge terminals ofbridge 41. The resonant circuits are each tuned to the line frequency.Each circuit does not yield a high power factor, but the combination ofthe two circuits yields a high power factor. The bulk and cost of thecomponents shown in FIG. 4 greatly exceeds the bulk and cost of theinductor and capacitor of the present invention as will be later seen.

The novel circuit of the present invention is illustrated in FIG. 5. InFIG. 5, the circuit includes a single phase bridge-connected rectifier50 (which could be any other desired type of rectifier) which has itsoutput terminals connected to an electrolytic filter capacitor 51. Theoutput of the filter capacitor 51 is then connected to a suitable d-cload represented by the load impedance 52.

In accordance with the invention, the relatively low frequency inputsource may be a 50 Hz or 60 Hz power source connected to terminals 53and 54. A tuned circuit consisting of inductor 55 and capacitor 56 isconnected in this input circuit on the a-c side of rectifier 50. Theinductor 55 and capacitor 56 are preferably tuned at a frequency ofabout three to six times the frequency of the low frequency sourceconnected to terminals 53 and 54. Typically, inductor 55 may have avalue of 30 millihenrys and capacitor 56 may have a value of 10microfarads, which yields a ratio of 4.8 to the line frequency of 60 Hz.This is to be contrasted to the values used for inductor 42 andcapacitor 43 in the prior art circuit of FIG. 4, which are typically 150millihenrys for inductors 42 and 44 and 50 microfarads for capacitors 43and 45 for the same load impedance.

Broadly stated, the operation of the circuit of the invention is suchthat, in the initial part of the half cycle of input voltage, when thevoltage across capacitor 51 exceeds the voltage on capacitor 56, therectifier 50 is reverse-biased and forces the line current to flow intocapacitor 56. The components 55 and 56 resonate, thereby causing linecurrent to flow as a short half cycle pulse in the initial portion ofthe half cycle of the input voltage until the voltage on capacitor 56exceeds the voltage on capacitor 51. The rectifier 50 becomesforward-biased and allows the line current to flow into capacitor 51 andthe load impedance 52. This provides a more uniform current flow in thea-c line over the one-half cycle increasing the duty cycle and improvesthe power factor of the circuit.

The operation of the circuit of FIG. 5 can also be understood from aconsideration of the voltage and current characteristics, shown as afunction of time, in FIGS. 6a, 6b and 6c. The characteristic current andvoltage in FIG. 6a is for a circuit of the type of FIG. 1 which issimilar to that of FIG. 5, but without the inductor 55 or capacitor 56.

In FIG. 6a, representing the line circuit of FIG. 1, the line current issharply peaked and the duty cycle is low so that the power factor of thecircuit is very poor. Conventionally the power factor of the circuit ofFIG. 1 will be of the order of 0.4 to 0.7. In addition, the line voltagein FIG. 6a is distorted and flattened during intervals corresponding tothe current peak.

If an inductor is added to the circuit in FIG. 1 as shown, for example,in FIG. 3, the voltage and current characteristics change to those ofFIG. 6b. In FIG. 6b, it will be seen that the current characteristic issomewhat less peaked than that of FIG. 6a and the conduction time islonger but the power factor of the circuit because of the displacementof the peak in the current with respect to the voltage is still poor andwould be from about 0.5 to 0.75 for a reasonably acceptable ripplefactor. However, the use of the relatively large inductor 34 createsmany other problems. Thus, the d-c output voltage is loweredconsiderably so that, for the same output power, additional current mustflow thereby resulting in reduced efficiency. Furthermore, the improvedpower factor is obtained at the expense of increasing the ripple in theoutput. Thus, if power factor is increased to about 0.9 when using thecircuit of FIG. 3, the ripple might be twenty percent notwithstanding aseverely reduced output voltage. A circuit of this type is shown in:

Peter Richman, Wave Factors and Power Factor for General Voltage andCurrent Waveforms. IEEE Transactions on Industrial Electronics andControl Instrumentation, Vol. IECI-22, No. 3, August 1975, pp. 420-424.

Peter Richman, Wave Factors for Rectifiers with Capacitor Input Filters,and Other High Crest-Factor Loads. IEEE Transactions on IndustrialElectronics and Control Instrumentation, Vol. IECI-21, No. 4, November1974, pp. 235-241.

The novel configuration of the present invention as shown in FIG. 5provides, through a series of complex and unexpected interactions,advantages considerably beyond those of the above-mentioned circuits.These can be explained with reference to FIG. 6c. Referring to thecentral curve in FIG. 6c, it will be seen that the current is drawn overvirtually all of the full half cycle as compared to the currents shownin FIGS. 6a and 6b. The line voltage at terminals 53 and 54 is shown inthe upper curve of FIG. 6c and approaches zero at approximately 170° inthe phase. At this point the voltage on capacitor 56 becomes less thanthe voltage on capacitor 51 so that the diode bridge 50 becomesreverse-biased. The current i shown in FIG. 5, therefore, can flow onlyinto capacitor 56. When the current reverses at about 180°, the currentcauses capacitor 56 to charge very suddenly in an opposite direction.Inductor 55 and capacitor 56 resonate at their period which is about 1/3to 1/6 of the period of the voltage of the line connected to terminals53 and 54. The resonant period will be approximately 2.8 to 5.5milliseconds as compared to the period of 16.6 milliseconds for the 60Hz sine wave. The sudden flow of charge into capacitor 56 causes thevoltage of capacitor 56 to swing sharply negative as shown in the lowercurve of FIG. 6c, exceeding the voltage on capacitor 51 and to againforward-bias the diode bridge 50. The current i continues to flow fromthe inductor 55 into both the capacitor 56 and capacitor 51 until nearlythe end of the half cycle at angle 350°. At that time, the diode bridge50 becomes reverse-biased, the current i flows into the capacitor 56 andits voltage reverses within the period set by the resonant frequency ofinductor 55 and capacitor 56. Thus, current conduction is affected overpractically all of both half cycles taken as an example and is in phasewith the a-c voltage, thereby giving good power factor with high outputvoltage and low ripple.

The following will also explain why the a-c current remains in phasewith the a-c input voltage, why the voltage across capacitor 56 is asquare wave in FIG. 6c, and the significance of these two factors.

The current i and the line voltage v are both near zero and the voltageof capacitor 56 has just passed its maximum value at 170°. The rectifierdiode bridge 50 is reverse-biased. The energy of the capacitor 56 whichis 1/2 CV² ₅₆ discharges into the inductor 55 so that at about 200° theenergy of the inductor (and its current) reaches its final maximum valueof 1/2 LI². The capacitor voltage passes through zero indicating thatthe capacitor is discharged. Because the capacitor 56 is disconnectedfrom the capacitor 51 and the load impedance 52 by the diode bridgerectifier, the ringing between capacitor 56 and inductor 55 has littleenergy dissipation. The inductor 55 then recharges the capacitor 56 toits full voltage in the reverse direction at about 230°. However, therectifier diode bridge 50 now becomes forward-biased. The inductorcurrent i, instead of reaching zero, now continues to flow into thecombination of the capacitors 56 and 51 and impedance 52 until itreaches 350°, when the process repeats.

By selecting the values for inductor 55 and capacitor 56 so that theenergy of inductor 55 is completely delivered to capacitor 56 at thetime that the rectifier diode bridge 50 becomes forward-biased, thecircuit requires no energy exchange with the line. That is, the storedenergy is exchanged between the inductor 55 and the capacitor 56 eachhalf cycle. This is also the condition for the input current and voltagefrom the lines 53 and 54 to appear at a unity power factor. The energyflow from the line just supplies the load. No energy is delivered to thecircuit which is stored in the inductor or capacitor and returned to theline at another point in the cycle.

When the rectifier diode bridge is forward-biased during each halfcycle, the capacitor 51 receives current i from the inductor 55. Thiscurrent replenishes the charge and energy of capacitor 51 that isrequired by the load impedance 52 over the entire half cycle. Sincecapacitor 51 is much larger than capacitor 56, the voltage of capacitor51 rises only slightly as it receives its energy. The result is that thevoltages on capacitor 51 and capacitor 56 during most of each half cycleremain nearly flat topped. Furthermore, the energy of the capacitor 51cannot be returned to the line during the half cycle, so that capacitor51 does not contribute to the energy exchange process between inductor55 and capacitor 56.

In summary, the desired current wave shape shown in FIG. 7 will beproduced when the resonant frequency of inductor 55 and capacitor 56 isabout 3 to 6 times the frequency of the input line. This wave form givesgood power factor. By contrast, if the resonant frequency is higher thanabout six times the line frequency, the current wave shape will be thatshown in FIG. 8 in which the duty cycle is not high enough, and willhave a poor power factor. If the resonant frequency is lower than aboutthree times the line frequency, the current wave shape will be thatshown in FIG. 9 which again has a low duty cycle and poor power factor.

To show that the current i is in phase with the line voltage v in thecircuit of FIG. 5, reference is made to the diagrams of FIGS. 10 and 11.Referring first to FIG. 10, the circuit is schematically illustrated asconsisting of the inductor 55, the capacitor 56 and the load resistor52. The inductor 55 is assumed to be 30 millihenrys. Capacitor 56 isassumed to be 10 microfarads and the load resistance R₁ of load 52 isassumed to be 43 ohms. The input impedance Z_(in) to the circuit of FIG.11 can be evaluated in terms of the fundamental component of the currenti using the impedance models Z_(L1) and Z_(p). The impedance Z_(p) is:##EQU3## where Z_(C1) is the reactive impedance of capacitor 56 at 60Hz.

Using the component values given above for the inductor 55, capacitor 56and load resistor 52 at 60 Hz, it can be shown that the rms current i is6.58 amperes with a phase angle of -6° with respect to the line voltage.This result indicates a power factor of about 0.995, which confirmsactual measurements on the circuit.

Analysis of the circuits of FIGS. 10 and 11 shows that close to unitypower factor can be obtained for a range of reactance ratios forinductor 55 from X_(L) /R=0.15 to X_(L) /R=0.4 by using resonantfrequencies for inductor 55 and capacitor 56 from 6.6 to 2.2 times theline frequency. The selection of the X_(L) /R reactance ratio dependsupon the ripple that can be tolerated on the voltage of capacitor 51,and the power factor requirements at less than rated load currents.

The above analyses shown that the current i has a long duty cycle, andis substantially in phase with the voltage v. Consequently:

1. An extremely high power factor is obtained.

2. The inductance 55 and capacitance 56 used in the circuit of FIG. 5are much smaller than similar components of comparable circuits. Thatis, the addition of the capacitor 56 reduces the needed energy storagein inductor 55 for a given load so that the size of the inductor 55 isconsiderably reduced.

3. The circuit of FIG. 5 operates without introducing high peak currentsbecause the input current i has a long duty cycle and is continuous.Thus line voltage waveform distortion is not introduced.

4. The maximum value of capacitor 51 is not critical because its valueis so much larger than that of capacitor 56 that its effect on thefurther increase of the output voltage during conduction of rectifier 50is insignificant. Consequently, relatively inexpensive electrolyticcapacitors can be used for the capacitor 51.

5. In-rush current is inherently limited because the inductor 55 is ofsuch a magnitude and is in series with the line that the peak value iscontrolled. Also, high power factor generally gives low in-rush currentand low transient voltages.

6. The output voltage of the circuit is not greatly attenuated becausethe value reached by the voltage on capacitor 56 depends on the valueschosen for the inductance 55 and capacitor 56. This is a result clearlyunobvious to those in this art.

7. Good power factor, for example greater or equal to 0.8, is retainedeven for wide variations, for example 3 to 1 in the resistance R₁ of theload 52. Therefore, the exact value of the load resistance is notcritical to the operation of the circuit.

8. Transient overvoltages from the line are suppressed before they candamage the semiconductor diodes of the rectifier 50.

9. The ripple output voltage is minimized because the capacitor 51 ismuch larger than the capacitor 56.

10. The circuit is inexpensive and extremely reliable because it usesall passive components of relatively small size and does not requirecomplicated switching and control schemes.

11. Good power factor is also retained for wide variations in dimming ofa given load. For example, when dimming from 100% to 20% of fullintensity, power factor goes from above 0.9 to about 0.8 (an acceptablepower factor).

FIG. 12 shows an embodiment of the invention which differs from that ofFIG. 5 but wherein the same theory of operation is used. In FIG. 12,power factor conditioning is done on the d-c side of the circuit ratherthan on the a-c side. Thus, the circuit has a single-phase and full-wavebridge-connected rectifier 70 connected directly to the terminals 53 and54 of the input a-c source. The tuned circuit consisting of inductor 55and capacitor 56 is connected across the d-c terminals of bridge 70 andthe output of the tuned circuit is connected to the a-c side of diode71. The output of the diode 71 is then connected to the filter capacitor51 and the circuit then is connected to the d-c load which is to beoperated from the power supply.

The circuit of FIG. 12 improves power factor by forcing current flow inthe initial portion of the half cycle and by causing the a-c current tobe drawn over virtually all of the full cycle. The theory of operationdisclosed above in connection with the circuit of FIG. 5 holds true forthe circuit of FIG. 12 in that, at 170° conduction time, the voltage oncapacitor 56 becomes less than the voltage on capacitor 51 so thatrectifier 71 now becomes back-biased. Thus, a-c current can now flowthrough the rectifier only into capacitor 56, thus giving capacitor 56 asudden charge and reversal of voltage. As before, this sudden chargefills in the initial portion of the a-c current waveform, making itcontinuous in nature throughout the cycle. Thus, the arrangement of FIG.12 differs from the arrangement of FIG. 5 only in that the input voltageis rectified before being conditioned by the tuned circuit consisting ofcomponents 55 and 56.

The operation of the circuit of FIG. 12 is shown in connection withFIGS. 13a to 13f in which the various voltage and current waveforms ofthe circuit are shown on a common time base. FIG. 13a shows the sinewave input voltage v which is connected to terminals 53 and 54. FIG. 13bshows the rectified voltage v' which is at the output d-c terminals ofthe bridge 70. The current flow i into the bridge 70 is shown in FIG.13c and is similar to the current waveforms in FIGS. 8 and 9. FIG. 13dshows the rectified current i' with regions at which diode 71 isreverse-biased being shown by circling. FIG. 13e shows the square waveoutput voltage v₅₆ which existed across capacitor 56 in the arrangementof FIG. 5 where the voltage is an a-c square wave. By comparison, theoutput voltage on capacitor 56 in FIG. 12 is a rectified square wave asshown in FIG. 13f. As can be seen from the waveforms of FIGS. 3a to 13f,all of the advantages described previously in connection with thecircuit of FIG. 5 hold for the circuit of FIG. 12. The circuit of FIG.12, however, has three additional advantages over the arrangement inFIG. 5:

1. The inductor 55 is in the d-c part of the circuit and thus will havesignificantly less core loss since there is a smaller a-c component offlux excursion. Thus, the circuit will be more efficient.

2. The inductor 55 will be much quieter than its a-c counterpart in FIG.5.

3. The inductor 55 can be a "swinging choke" which further improvespower factor during dimming. A swinging choke is a non-linear inductorwhose inductance increases as current decreases. The effect of thisincreasing inductance will compensate for a capacitive effect thatoccurs during dimming or reduction of load current.

4. The inductor 55 can incorporate a permanent magnet to bias themagnetic circuit against the direct current and thus reduce its size.

The above advantages are, of course, offset by the need for theadditional diode or rectifier element 71.

The circuits of FIG. 14 to 19 which are to be described hereinafter allinclude features to maintain a high power factor (greater than about0.9) throughout a wide range of variation of load current as due todimming of a lamp load. The circuit of FIG. 14 is very similar to thatof FIG. 5 and similar numerals identify similar components in the twocircuits.

In the arrangement of FIG. 5, as the system is dimmed or load current isdecreased, the power factor decreases somewhat since the circuit becomessomewhat capacitive. The peak energy of capacitor 56 exceeds the peakenergy of the inductor 55. To counteract this effect, the circuits ofFIGS. 14 and 15 provide a current controlled inductor arrangement. Thus,in FIG. 14 an inductor 80 is provided where the inductor 80 has a d-cbiasing winding 81 and a biasing source 82 which can be operated to varythe inductance of the inductance coil 80. Thus, in the system of FIG.14, the inductance of member 80 can be more or less varied in order tocounteract the effect of added capacitance during dimming. If desired,the control which regulates load current can simultaneously adjust thebias 82 to obtain the desired compensation.

In FIG. 15, the inductor 55 of FIG. 14 is replaced by a currentcontrolled inductor 90 which acts both as the resonating inductor 55 inFIG. 5 and as the compensating inductor 80 of FIG. 14 in a singlecomponent. Note that in FIG. 15 the control winding 91 of the inductor90 is connected to a variable source of a-c controlled power 92 in theusual manner well known to those skilled in the art.

The circuits of FIGS. 16 and 17 are similar to those of FIGS. 5, 14 and15 and like components have received similar identifying numerals. Inthe circuits of FIGS. 16 and 17, however, the capacitive current duringdimming or reduction of output current is compensated by the addedparallel inductor 100 which can be connected either in front of orbehind the resonant circuit consisting of inductor 55 and capacitor 56(FIGS. 16 and 17, respectively).

FIGS. 18 and 19 show a circuit arrangement wherein a portion of thecapacitance of the capacitor in the resonant circuit is switched in andout of the circuit for power factor correction purposes. Again, in FIGS.18 and 19, components identical to those of FIG. 5 have been givenidentical numerals. However, the output load circuit in each of FIGS. 18and 19 is schematically illustrated as the adjustable load 110 whichproduces some output of any desired type for turning on a triac 111 orother suitable switching means when the output current is greater thanabout 50% of its rating.

In FIG. 18, the capacitor 56 of FIG. 5 is replaced by twoseries-connected capacitors 112 and 113 and the capacitor section 113 isin parallel with the triac 111 so as to be shorted by the triac when thetriac 111 is on. Thus, when the output current of the power supplyexceeds some given value, the total capacitance in resonance with theinductor 55 is equal to that of capacitor 112. However, when the loadcurrent reduces below 50%, for example, capacitor 113 is switched intothe circuit in series with capacitor 112, thereby to decrease the totalcapacitance which resonates with inductor 55. This will maintain arelatively high power factor for the circuit under a high dimmingcondition as where the circuit is used for the control of lamps asdisclosed in previously mentioned co-pending application Ser. No.966,604, filed Dec. 5, 1978.

The circuit of FIG. 19 operates in a substantially identical mannerexcept that the capacitors which resonate with inductor 55 areparallel-connected capacitors 114 and 115. Capacitor 114 is in serieswith the triac 111 so that when current output to load 110 is greaterthan about 50%, capacitors 114 and 115 are connected in parallel and thetotal capacitance resonating with inductor 55 is the sum of thecapacitances of capacitors 114 and 115. The capacitor 114 is removed,however, when triac 111 turns off in order to obtain an appropriateadjustment of the power factor of the circuit.

Although there have been described preferred embodiments of thisinvention, many variations and modifications will now be apparent tothose skilled in the art. Therefore, this invention is to be limited,not by the specific disclosure herein, but only by the appended claims.

What is claimed is:
 1. A high power factor power supply circuitcomprising, in combination:an a-c supply circuit having a relatively lowa-c frequency; a tuned circuit comprising an inductor and capacitorhaving respective values which are tuned to resonate at a frequencywhich is higher by less than about one order of magnitude than saidrelatively low a-c frequency; coupling means for connecting said a-csupply circuit to said tuned circuit; a rectifier means having a-c inputmeans connected to said tuned circuit and having a d-c output circuitmeans; said inductor being connected in series with said rectifiermeans; said capacitor being connected in shunt with said rectifier meansand having one terminal connected to the junction between said inductorand said rectifier means; and an output capacitor connected to said d-coutput circuit means.
 2. The circuit of claim 1 which further includesload circuit means connected to said d-c output circuit means; said loadcircuit means drawing variable current from said a-c supply circuit. 3.The circuit of claim 1 wherein said rectifier means comprises a singlephase, full-wave bridge-connected rectifier; and wherein said couplingmeans includes connection wires for connecting said a-c supply circuitto said inductor and capacitor respectively.
 4. The circuit of claim 1wherein said a-c supply circuit has a sinusoidal voltage and a frequencyof 50 Hz to 60 Hz.
 5. The circuit of claim 1 wherein said tuned circuithas a resonant frequency of about 3 to 6 times said relatively lowfrequency.
 6. The circuit of claim 1 wherein said coupling meansincludes a second rectifier means.
 7. The circuit of claim 1 whereinsaid output capacitor is an electrolytic capacitor.
 8. The circuit ofclaims 1, 2, 3, 4, 5, 6 or 7 wherein the wave shape of the current drawnfrom said a-c supply is approximately in phase with the voltage thereof,and wherein said current has a long duty cycle.
 9. The circuit of claim2 which further includes variable inductance means connected in saidcircuit to compensate for increased capacitive reactance in said circuitwhen output d-c current is reduced.
 10. The circuit of claim 2 whichfurther includes variable capacitance means for changing the capacitanceof said capacitor as the output d-c current is reduced.
 11. A powersupply circuit having a high power factor; said circuit comprising, incombination:input a-c power terminal means for connection to an a-csource having a given frequency; a rectifier means having a-c and d-cterminals; an output capacitor connected across said rectifier means d-cterminals; an inductor connected between one of said a-c power terminalsand one of said a-c terminals of said rectifier means; a secondcapacitor connected across said rectifier means a-c terminals; saidinductor and said second capacitor being resonant at a frequency whichis a multiple of about three to six times said given frequency; saidoutput capacitor being substantially larger than said second capacitor.12. The power supply of claim 11 which includes second rectifier meansconnected directly to said a-c power terminal means for supplyingrectified power to said inductor and said second capacitor.
 13. Thepower supply circuit of claim 11 or 12 which further includes loadcircuit means connected across said output capacitor for drawing avariable current from said a-c power terminal.
 14. The power supplycircuit of claim 11 or 12 wherein said rectifier means is a singlephase, full-wave bridge-connected rectifier.
 15. The power supplycircuit of claim 12 wherein said rectifier means is a diode and whereinsaid second rectifier means is a single phase bridge-connectedrectifier.
 16. The power supply circuit of claim 11 which furtherincludes a second inductor connected across said a-c power terminalmeans.
 17. The power supply circuit of claim 11 which further includes asecond inductor connected across said a-c terminals of said rectifiermeans.
 18. A high power factor supply circuit comprising:a-c supplycircuit means having any particular frequency; tuned circuit meansincluding a capacitor means and inductor means resonant with one anotherat a frequency of from about three to six times said particularfrequency; rectifier means having a-c input means connected to saidtuned circuit means and having d-c output means; second capacitor meansconnected to said d-c output means; and output circuit means connectedacross said second capacitor means.
 19. The circuit of claim 18 whereinsaid inductor means, capacitor means and second capacitor means haverespective values that have such relation to each other that the currentof said a-c supply circuit means has an increased magnitude in the firstportion of each half cycle.
 20. The circuit of claim 18 wherein saidrectifier means comprises a single phase, full-wave bridge-connectedrectifier; and coupling means including connection wires for connectingsaid a-c supply circuit means to said inductor means and capacitor meansrespectively.
 21. The circuit of claim 18 wherein said a-c supplycircuit means has a sinusoidal voltage and a frequency of 50 Hz to 60Hz.
 22. The circuit of claim 18 wherein said second capacitor means isan electrolytic capacitor.
 23. The circuit of claims 18, 19, 20 or 21wherein the wave shape of the current drawn from said a-c supply circuitmeans is approximately in phase with the voltage thereof, and whereinsaid current has a long duty cycle.